At least after a period of time or after somebody replied to it you cannot remove your posts.
In general, this is a bad idea anyway, since it makes a thread unreadable. It is no disaster if somebody would post a wrong answer to a thread; removing that answer after somebody else replied to it would make it very hard for other people to understand what the posts are all about.
If however you feel you posted something that really needs removing, for example because you posted some classified info, or you insulted somebody or whatever, then you could report your message, or ask one of the moderators to remove it.